The present invention relates generally to the field of seals used in turbomachinery, and in particular to a compliant seal for application at the interface of a rotating component, such as the rotor blades in a turbine, and a stationary component, such as a shroud in a turbine.
A number of applications call for sealing arrangements between rotating and stationary components. Such seals may vary in construction, depending upon such factors as the environments in which they function, the fluids against which they form a seal, and the temperature ranges in which they are anticipated to operate. In turbine and similar applications, for example, seals are generally provided between the various stages of rotating components, such as turbine blades, and corresponding stationary structures, such as housings or shrouds within which the rotating components turn.
Efficiency and performance of gas and steam turbines are affected by clearances between rotor blade tips and the stationary shrouds, as well as between the nozzle tips and the rotor. In the design of gas and steam turbines, it is desirable to have a close tolerance between the tips of the rotor blades and the surrounding static shroud. In a turbine stage, any portion of the working fluid passing through the clearance between the tips of the rotor blades and the static shroud does no work on the rotor blades, and leads to a reduced work efficiency of the turbine stage. Likewise, such leakage in a compressor stage leads to a reduced compression efficiency. Generally, the closer the shroud or stationary component surrounds the tips of the rotor blades, the greater is the efficiency of the turbomachinery.
However, the clearance dimensions between the rotor blade tips and the shroud vary during various operating modes of the turbine engine. A significant reason for this is the dissimilar thermal growth within the engine between the blade tips of the rotor and the shroud surrounding them. In such a case, the high temperature of the working fluid causes a thermal discrepancy between the shroud and the rotor blades, wherein the shroud is at a lower temperature than the rotor blades. The time interval until the thermal equivalence between the shroud and the blades is restored may be referred to as the transient period. The clearance between the shroud and the blades decreases during this transient period as the components reach their steady state conditions and dimensions, and causes the interfacing surfaces to rub, thereby leading to rapid wear of the blade material.
Mechanical and aerodynamic forces can also affect the clearance at the interface of a static shroud and the rotor blade tips. Clearance changes in certain turbine configurations may result from the turbine passing through critical speeds. Compliant seals may be needed to accommodate such clearance changes. This can also lead to interference between rotor blades and the shroud. In certain applications, such as aircraft engines, mechanical forces during operation (such as take-off and landing) can result in similar clearance changes.
Prior methods to solve the above problems include using a seal on the stationary shroud surface, the sealing material being designed to be wearable or abradable with respect to the rotor blade rubbing against them. In such a system, during the transient period, the blade tip contacts or rubs against the shroud, causing the shroud material to abrade or flake off. This avoids damage to the rotating elements, and provides reduced clearances and thus better sealing compared to a non-abradable system, in which large cold-built clearances have to be provided to prevent rubbing during the transient period. However, this abradable system suffers from the disadvantage of reduced life of the sealing material. Also, previous abradable seals, even though various materials for the shroud have been proposed such as sintered metal, metal honeycombs and porous ceramics, have not provided a desirable flexibility. After wear due to a transient condition, such as a thermal transient or shock loading, the gap or wear produced by the rub or contact is larger than the interference depth, due to tearing out, galling and spalling.
A different approach is also known in the art, and involves the use of brush-seals on the inner surface of the stationary shroud. One implementation of brush-seals involves the use a plurality of bristle packs supported on the inner periphery of the stationary shroud, such that the bristle distribution at the inner diameter of the bristle packs is substantially continuous. The inner diameter of the bristle packs can serve as a moderately compliant surface, which, due to the bending of the bristles, is compressed radially outwards when the shroud-rotor blade clearance decreases. However, while brush seals generally work satisfactorily for steam turbines, and for compressor stages, where temperatures do not exceed 650 to 700 degrees centigrade, they may not be always suitable for first or second stage gas turbines, where the temperature can exceed 800 degrees centigrade. Furthermore, such seals have an inherently porous nature due to their fibrous structure, and lead to leakage of the working fluid. Brush seals are also known to be subject to wear due to the continuous rubbing between the bristles and the blade tips. Moreover, the intermittent nature of certain shrouds may result in problems with bristles of such brush seals as the shroud crevices are contacted by the bristles.
There is a need, therefore, for a sealing device, for application at the interface of the rotating components, such as the blade tips of a turbine or compressor, and the stationary components, such as a shroud, which would substantially comply with the relative changes in radial clearance between the blade tips and the shroud, so as to minimize the damage caused to the rotating parts, as well as the wear on the shroud-tip material. There is further a need for a sealing device which is capable of withstanding the high temperatures in different steam or gas turbine stages, while effectively reducing the leakage of the working fluid, so as to improve the engine efficiency.